1. Field of the Invention
The present invention relates to load bearing in the construction of buildings and in particular to a shear lug for transmitting shear forces from framing elements to a foundation on which the framing elements are supported.
2. Description of the Related Art
Framing elements used for example in lightweight constructions are mounted to foundations so as to resist a variety of forces. An example of a framing element 20 is shown in prior art FIG. 1 mounted to a foundation 22. Lateral loads, L, may be exerted on the framing element 20, for example upon seismic activity or winds. The lateral loads L as shown generate tensile forces T and compressive loads C on respective columns 24 of the framing element 20 as shown. In order to transfer the tensile loads from the column 24 to the foundation 22, anchor rods 30 are bolted to the column base and extend through the base down into the concrete foundation.
In addition to the tensile and compressive forces, lateral loads may also generate shear forces, S, transverse to the length of the column at the column base. In some constructions, the frictional forces generated by the axial compressive loads on the frame columns are sufficient to oppose the shear forces. However, for constructions bearing higher shear forces, a variety of structures and methods are known for transferring these shear forces to the foundation. Such structures and methods include embedding the column itself into the foundation and providing anchor bolts to provide a clamping force resisting shear loads.
A third alternative is to provide a shear lug mounted to the base of the frame column. Prior art FIG. 2 shows a shear lug 40 mounted to the base plate 42 of a column 24 and positioned in the foundation 22. The shear lug 40 is in general a plate, or fin, welded perpendicularly to the bottom of the base plate 42. In practice, the base plate and shear lug are first bolted to the bottom of the column. A trench, or key, 46 is then formed in the foundation having a depth and width larger than the height and width of the shear lug. The base plate is then positioned atop the foundation, with the shear lug positioned within the key. A layer of grout 48 is provided to fill the key and a space between the base plate and foundation. With this structure, shear force is transferred from the column base, through the base plate and shear lug, into the grout and foundation, with the shear lug acting as a cantilever to transfer shear down into the foundation.
The use of a shear lug in this conventional manner has certain drawbacks. For example, fitting the shear lug to a preformed key in the foundation weakens the foundation and reduces the ability of the foundation to absorb the applied shear forces. At times, a wedge of the foundation can shear off, especially where the column and shear lug are close to an edge of the foundation. Additionally, the weld of the shear lug to the base plate is subject to high stresses and can at times fail under high shear loads.